Mars - the red planet

Mars is often called the 'Red Planet' because it appears in the sky as an orange-red star. The colour caused the ancient Greeks and Romans to name it after their god of war. Today, thanks to visiting spacecraft, we know that the planet's appearance is due to rust in the Martian rocks.

Mars is the fourth planet from the Sun. It orbits the Sun at an average distance of 228 million km, half as far again as the Earth, so human visitors would find it very cold. Although summers near the equator can be quite warm, the average temperature is 63 degrees Celsius below zero - similar to winters in Antarctica. The nights are also bitterly cold.

The first humans on Mars will have other problems to face. The air is 100 times thinner than on Earth, and mostly made up of carbon dioxide. Human explorers will have to wear oxygen masks and special suits every time they step outside their sealed homes.

Violent storms can whip up clouds of dust. Sometimes these spread rapidly around the entire planet, hiding the surface from view.